1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, more particularly, to a method of manufacturing a bipolar transistor operating at a low temperature such as liquid nitrogen temperature (77 K).
2. Description of the Related Art
At a low temperature, the characteristics of MOS transistor is improved and wiring resistance is decreased. The decrease of wiring resistance is effective to improvement of operation speed of a bipolar LSI and a MOS LSI operating at low temperature such as liquid nitrogen temperature has been developed. In a BiCMOS LSI in which a bipolar transistor and a MOS transistor are formed on the same chip, the bipolar transistor needs to have a large drive capability because wiring capacitance is not decreased even at low temperature.
The characteristic of the bipolar transistor at low temperature is quite different from that of the bipolar transistor at room temperature. Phenomena such as bandgap narrowing and carrier freeze-out are caused at low temperature.
It is reported by Johannes M. C. Stork et al. in "Base Profile Design for High-performance operation of Bipolar Transistors at Liquid-Nitrogen" that the carrier freeze-out phenomenon is caused in semiconductor at low temperature so that the resistivity increases if the doping level is less than 1.times.10.sup.18 cm.sup.-3. In order to prevent the carrier freeze-out phenomenon, the doping level must be high. On the other hand, when the doping level is high, the amount of bandgap narrowing is increased at low temperature. In an n-p-n type of transistor, in a case where the acceptor doping level is high in the base layer, when the emitter layer is formed in the base layer through impurity diffusion, the acceptor doping level in the emitter layer is increased, so that the amount of bandgap narrowing in the emitter layer is larger because of the donor doping level and the diffused acceptor doping level than that estimated based on only the donor doping level. As a result, the difference of bandgap narrowing amount between the emitter layer and the base layer becomes great so that h.sub.FE is decreased with temperature becomes lower.
Even in a conventional transistor structure, if the doping level in the base layer is increased to that in the emitter layer, the reduction of h.sub.FE at low temperature can be prevented because the difference between the emitter layer and the base layer in the bandgap narrowing amount can be made small. This is also reported in the above paper by Stork et al. The inventor of the present invention fabricated and examined samples based on the teaching of the above paper. In the paper, the emitter layer is formed using standard LPCVD polysilicon deposition, arsenic ion implantation and diffusion. In the samples, an n.sup.+ -type buried layer and an n-type epitaxial layer (a collector layer of a bipolar transistor) are formed on a p-type silicon substrate and a base layer is formed which is composed of a p-type epitaxial layer selectively grown on the epitaxial layer at a low temperature in a range of 450.degree. to 700.degree. C. The impurity distribution profile of one of the samples is shown in FIG. 1. The doping level of n-type impurity in the emitter layer is 1.times.10.sup.20 cm.sup.-3 and the base layer has the boron doping level of 2.times.10.sup.19 cm.sup.-3 and 55 nm in thick. If the base layer is formed using ion implantation, the impurity distribution becomes so wider that a low doping level region is formed. As a result, carrier are frozen out in such a low doping level region, resulting in increased base resistivity. Since the bandgap narrowing amount is influenced by both the donor doping level and the acceptor doping level in a semiconductor, the actual amount .DELTA.E.sub.ge is greater than that estimated in consideration of only the donor doping level. Thus, when the bandgap narrowing amount .DELTA.E.sub.ge in the emitter layer having a high doping level is greater the bandgap narrowing amount .DELTA.E.sub.gb in the base layer having a low doping level, the number of minority carriers injected from the base layer to the emitter layer would increase with temperature being decreased. In the samples, when the bandgap narrowing amounts are measured, .DELTA.E.sub.ge was 96 meV and .DELTA.E.sub.gb was 95 meV. This is reported in "Measurement of Steady-state Minority-Carrier Transport in Heavily doped n-Type Silicon" (IEEE Transactions on Electron Device, Vol. ED-34, pp. 1580-1589, 1987) by J. A. Del Alamo et al. and in "Measurement of Electron Lifetime, Electron Mobility and Band-gap Narrowing in Heavily Doped p-type silicon" (Digest of International Device Meeting, PP. 24-27, 1986) by S. E. Swirhun et al.
The dependency of h.sub.FE upon temperature of the samples was measured. The measuring result is shown in FIG. 17 and h.sub.FE is decreased with temperature being decreased. Also, as shown in FIG. 2, the cut-off frequency f.sub.T is decreased at low temperature (89 K) compared to that at room temperature (300 K). This is because the emitter traveling time increases due to the decreased h.sub.FE. In this case, h.sub.FE is expressed by the following equation (1) EQU h.sub.FE =(W.sub.E .multidot.N.sub.E .multidot.D.sub.nB /W.sub.B .multidot.N.sub.B .multidot.D.sub.pE)exp {(.DELTA.E.sub.ge -.DELTA.E.sub.gb)/kT{ (1)
where N.sub.E is a doping level in the emitter layer, N.sub.S is a doping level in the base layer, D.sub.pE is a diffusion coefficient of a hole in the emitter layer, D.sub.nB is a diffusion coefficient of an electron in the base layer, W.sub.E is a thickness of the emitter layer, W.sub.B is a thickness of the base layer, k is a Boltzmann constant, and T is an absolute temperature. The great decrease of h.sub.FE at low temperature makes the emitter traveling time increase, resulting in the decreased cut-off frequency f.sub.T.
Recently, in order to solve the problems, there have been proposed bipolar transistors having new structures. For instance, a heterojunction bipolar transistor in which material having a narrow bandgap such as silicon-germanium is used for the base layer, is proposed as the first example in "Low Temperature Operation of Si and SiGe Bipolar Transistor" (IEEE IEDM Technical Digest, pp. 17-20, 1990) by E. F. Grabbe. Although this transistor shows high h.sub.FE even at low temperature, f.sub.T abruptly decreases in a high collector current region due to the heterojunction between the base and the collector, so that the transistor cannot be used in a circuit operating in a high current region such as BiCMOS. This is reported in "Profile Scaling Constraints for Ion-Implanted and Epitaxial Bipolar Technology Designed for 77 K Operation" (IEEE IEDM Technical Digest, pp. 861-864, 1991) by J. D. Cressler et al.
As the second example, a pseudo-heterojunction bipolar transistor (HBT) having an emitter doping level lower than a base doping level is proposed in "Base-Emitter Injection Characterization in Low-Temperature Pseudo-Heterojunction Bipolar Transistor" (IEEE Transactions on Electron Device, vol. 37, No. 10, pp. 2222-2229, 1990) by K. Yano et al. In this case, the bandgap narrowing amount .DELTA.E.sub.gb of the base layer having a high doping level is greater than that .DELTA.Eg.sub.e of the emitter layer having a low doping level, so that h.sub.FE is improved. However, in this example, because the emitter doping level needs to be lower than that in a conventional transistor, there is caused another problem of emitter resistance increasing.
The conventional bipolar transistors studied for low temperature operation was described. A conventional bipolar transistor not for low temperature operation (JP-A-Hei4-99328) will be described with reference to an impurity distribution profile shown in FIG. 3. After a n.sup.+ -type buried layer and a n-type epitaxial layer (a collector layer of a bipolar transistor) are formed on a p-type silicon substrate, a p-type base layer is formed using an ion injection method (or an MBE method). FIG. 3 shows the impurity distribution profile of a bipolar transistor using the ion injection method. This base layer includes a first base layer having a high peak doping level of 1.times.10.sup.18 to 5.times.10.sup.18 cm.sup.-3 and a second base layer having a low peak doping level of 2.times.10.sup.16 to 1.times.10.sup.17 cm.sup.-3. An n.sup.+ -type emitter layer having a surface doping level of 1.times.10.sup.20 to 1.times.10.sup.21 cm.sup.-3 is formed on a part of the second base layer. As shown in FIG. 3, because the emitter layer of the high doping level does not contact the first base layer of high doping level, an electric field in a depletion layer when a backward voltage is applied between the emitter layer and the base layer is weakened so that the generation of hot carries is suppressed. However, in this example, because the total thickness of the base layers is thick due to the second base layer so that the base traveling time required for electrons to pass through the base layers becomes long, the cut-off frequency f.sub.T decreases. In addition, an electric field is generated between the first and second base layers because of the difference therebetween in the doping level. This electric field acts to suppress the electron traveling in the base layers to increase the base traveling time. Further, because the second base layer is of a low doping level, the resistance of an intrinsic base layer below the emitter layer increases due to the freeze-out at low temperature.
A bipolar transistor in which the low temperature operation is considered is disclosed in JP-A-Hei5-129315. FIGS. 4 and 5 show the cross sectional view and the impurity distribution profile of the transistor. This transistor has an inversely graded base impurity distribution profile in which the doping level of a base layer on a collector layer side is higher than that on an emitter layer side. The base doping level N.sub.BC on the collector layer side is 3.times.10.sup.19 cm.sup.-3 and the base doping level N.sub.BE on the emitter layer side is 3.times.10.sup.18 cm.sup.-3 . When a base layer width is 100 nm, an inverse electric field, i.e., an electric field which acts to back electrons from the collector layer side to the emitter layer side in the base layer is generated due to the difference in pseudo-fermi levels produced from the difference in the doping level. In this case, the electric field E1 is expressed by the following equation (2). ##EQU1## where T is an absolute temperature (K), k is the Boltzmann constant (1.38.times.10.sup.-23 J/K), and q is a charge an electron (1.6.times.10.sup.-19 C). E1 is 2,331 KV/cm at room temperature and 0.598 KV/cm at liquid nitrogen temperature. On the other hand, a forward electric field E2 is generated because of the difference between the collector and emitter layer sides in the bandgap narrowing amount which is generated due to the base layer doping level distribution, to accelerate electrons from the emitter layer side to the collector layer side in the base layer. The bandgap narrowing amounts .DELTA.E.sub.gb1 and .DELTA.E.sub.gb2 are respectively 103 meV and 62 meV for N.sub.BC =3.times.10.sup.19 cm.sup.-3 and 3.times.10.sup.18 cm.sup.-3 and in this case E2 is determined as follows. ##EQU2## where the bandgap narrowing amount of the conduction band in which electrons travel is assumed to be a half of the total bandgap narrowing amount. E2 is not dependent upon temperature and is 2.05 KV/cm. For this reason, E1 is greater than E2 at room temperature so that the inverse electric field acts against electrons. However, E1 becomes small as temperature falls and the forward electric field acts to accelerate electrons at 264 K or below, so that electron base traveling time is shortened to increase the cut-off frequency f.sub.T. "Ultra high speed bipolar device", T Sugano, p. 37, published by Baihuukan is referenced to for the equation (2) and "Reduction of f.sub.T by Nonuniform Base Bandgap Narrowing" by S. Szeto et al. (IEEE Electron Device Letters, Vol. 10, pp. 341-343, 1989) is referenced to for the equation (3). The bandgap narrowing amount of the base layer is calculated based on the above "Ultra High Speed Bipolar Device".
In a recent high speed bipolar transistor, high concentration impurity is doped in the collector layer to suppress Kirk effect due to which f.sub.T is dropped at a high current region. However, if the entire collector layer is highly doped, a junction capacitor between the collector layer and the base layer increases to prevent the high speed operation. For this reason, Selectively Ion Implanted Collector (SIC) structure is employed in which ion implantation is selectively performed for the collector layer straightly below the emitter layer to form a high doping level region.
Even in the above low temperature operating semiconductor device disclosed in the JP-A-hei5-129315, the SIC structure is required in order to suppress the Kirk effect. Generally, the ion implantation for the SIC structure is performed after the base layer is formed, as disclosed in JP-A-Sho63-107167. As a result of our experiment, if ion implantation is performed after the base layer having the inversely graded impurity distribution is epitaxially grown, i.e., if the collector layer impurity is highly injected in the base layer, the improvement of f.sub.T at low temperature is almost not achieved unlike disclosure of JP-A-Sho63-107167. The impurity distribution profile of a transistor used in this experiment is shown in FIG. 6 and the temperature dependencies of f.sub.T and h.sub.FE are shown in FIGS. 7 and 8. In a transistor in which the SIC injection is performed after the base layer is epitaxially grown, f.sub.T and h.sub.FE at 77 K drop compared to those at room temperature regardless of impurity distribution profile, i.e., even in a box-shape impurity profile and even in the inversely graded impurity distribution profile. This is because if phosphorus ions are injected in the base layer containing boron ions as impurity for the SIC structure, the phosphorus ions form a donor level to trap electrons traveling in the base layer. This phenomenon becomes remarkable as temperature drops so that f.sub.T and h.sub.FE are fallen down. For this reason, the inversely graded impurity distribution effect is almost hidden at low temperature.
Another method of forming the SIC structure is disclosed in JP-A-Hei4-315438 which relates to a bipolar semiconductor integrated circuit device. The low temperature operation is not considered in this device. In this method, after the SIC structure is formed, the base layer is formed using epitaxial growth method, the degradation of crystallization of the base layer due to the SIC injection can be prevented.
As disclosed in the JP-A-Hei4-315438, the drop of f.sub.T and h.sub.FE due to the trapping is not caused at low temperature when the donor level is not formed in a case of transistor which the SIC injection is performed prior to the base epitaxial growth, i.e., impurity for the collector layer is not injected into the base layer. The impurity distribution profile of the transistor used in the test is shown in FIG. 9 and the temperature dependencies of f.sub.T and h.sub.FE are shown in FIGS. 10 and 11. In the transistor having a conventional box-shaped impurity distribution profile in which the SIC injection is performed after the base layer is formed, h.sub.FE decreases as the temperature decreases. However, in a transistor in which the SIC injection is performed before the base layer is formed, the drop of h.sub.FE is suppressed in a great extent even at 77K. The same result is obtained with respect to f.sub.T. However, the low temperature operating transistor cannot be achieved by only the performing the SIC injection before the base layer is formed.
In the JP-A-Hei5-129315, a polysilicon layer is formed on the epitaxial base layer. When the polysilicon layer is etched, the base layer would also be etched. Therefore, in order to increase f.sub.T, it would be difficult to make the base layer thin. In addition, in the JP-A-Hei4-315438, the transistor structure expands in a lateral direction. Therefore, the scaling in the lateral direction would be difficult.